1. Field of the Invention
This invention relates to a method of forming high T.sub.c superconducting films by organometallic chemical vapor deposition. More specifically, this invention relates to a method of forming superconducting films on a semiconductor surface by chemical vapor deposition of volatile organometallic compounds of the rare earth and alkaline earth elements, and copper.
2. Description of Prior Art
The discovery of high temperature superconductivity in ceramic oxides with the perovskite structure has generated considerable research in fabricating thin films of these materials by a variety of techniques. The synthesis of bulk powder samples of ceramic metal oxides that are superconducting, such as YBa.sub.2 Cu.sub.3 O.sub.6.5, has been accomplished by heating stoichiometric mixtures of the individual metal oxides. The resulting powders, however, are not suitable for forming thin films of material that might be used in electronic devices.
Several techniques have been proposed for making thin films of superconducting materials on a substrate including spray pyrolysis, sputtering, evaporation, laser ablation, and spin coating. See R. B. Laibowitz, R. H. Koch, P. Chaudhari, and R. J. Gambino, Phys. Rev. B. 35, 8821 (1987); M. Hong, S. H. Liou, J. Kwo, and B. Davidson, Appl. Phys. Lett., 51, 694 (1987); J. Kwo, T. C. Hsieh, R. M. Fleming, M. Hong, S. H. Liou, B. A. Davidson, and L. C. Feldman, Phys. Rev. B, 36, 4039 (1987); X. D. Wu, D. Dijkkamp, S. B. Ogale, A. Inam, E. W. Chase, P. F. Miceli, C. C. Chang, J M. Tarascon, and T. Venkatesan, Appl. Phys. Lett., 51, 861 (1987); A. H. Hamdi, J. V. Mantese, A. L. Micheli, R. C. O. Laugal, D. F. Dungan, Z. H. Zhang, and K. R. Padmanabhan, Appl. Phys. Lett., 51, 2152 (1987); A. Gupta, G. Koren, E. A. Giess, N. R. Moore, E. J. M. O'Sullivan, and E. I. Cooper, Appl. Phys. Lett., 52, 163 (1988). These materials show promise for use in both large and small scale applications if a process is developed for depositing this class of superconductors on a variety of substrate geometries compatible with state-of-the-art semiconductor growth and processing techniques.
Chemical Vapor Deposition (CVD) processes have been used to make semiconductors. A matter of primary concern in a CVD process is the selection of precursors with suitable transport properties. In the case of metal oxide films, metal alkoxides, M(OR).sub.x, and metal .beta.-diketonates, M(RCOCHCOR).sub.x, are known to have volatility. H. Prakash, Prog. Crystal Growth and Charact., 6, 371 (1983). The alkoxides are recognized to be moisture sensitive. The copper precursor, copper acetylacetonate, Cu(CH.sub.3 COCHCOCH.sub.3).sub.2, is known to be commercially available. Its transport properties are not well understood, but there is at least one report of its use in the growth of CuO films. L. A. Ryabova, Current Topics in Material Science, Vol. 7, edited by E. Kaldis (North-Holland, Amsterdam, 1981), pp. 598-642. The corresponding yttrium acetylacetonate is reported to be nonvolatile, whereas other yttrium chelates with fluorinated and non-fluorinated groups substituted in the 1 and 3 positions of the .beta.-diketone ligand are described as volatile. E. W. Berg and J. J. Chiang Acosta, Anal. Chim. Acta, 40, 101 (1968)
At page 141-143 of the Extended Abstracts, High-Temperature Superconductors II, edited by Capone et al. and distributed Apr. 5-9 1988, at the Symposium Support Department of Energy Division of Material Science, at Reno, Nev., Berry et al. disclosed a formation of High T.sub.c superconducting films by organometallic chemical vapor deposition (OMCVD) utilizing .beta.-diketonates. About May 26, 1988, Nakamori, Abe, Kanamori, and Shibata distributed a paper directed to the same subject matter. Nakamori et al. noted problems with the stability of the .beta.-diketone chelate and with converting the films to superconductors. Nakamori et al. suggested trying the fluorinated analogs.
The diketone chelates are known to hydrolyze upon heating. Charles et al., J. Inorg. Nucl. Chem., 26, pp. 373-376, (1964); Springe et al., Inorganic Chem., Vol 6, #6, pp. 1105-1110, June 1967; Richardson, Inorganic Chem., Vol. 10, #3, pp. 498-504 (1971); Cranley et al., Anal. Chim., 60, pp. 109-116, (1972); Berg et al., Anal. Chim., 60, pp. 117-125 (1972). The hydrolyzed product will block feed passages used in a CVD process if they form during transport and will not be volatile if they form before transport.